PL / SQL游标

Whenever you try to execute an SQL statement, Oracle processes it in a separate memory zone called as ‘Context Area’. It contains information regarding information retrieved from database tables/records, SQL query, number of rows accessed by it and many more. This Context Area is controlled by a ‘Cursor’. A PL/SQL cursor can hold one or more rows returned by an SQL statement query. The set of rows that a Cursor holds at a single point of time is called as an ‘Active Set’.

A Cursor is basically a pointer to the Context Area. PL/SQL block controls the context area using a cursor. A cursor holds the one or more rows returned by a PL/SQL statement. Cursors and can be named as well as be kept anonymous. Named cursors can help for its reference so that it can be fetched in a program later on and process the rows returned by the SQL statement one by one. 

In Oracle SQL, there are two types of cursors:

1. Implicit cursors 2. Explicit cursors

Implicit Cursors

The Implicit Cursor is the Default Cursor in PL/SQL blocks. These are created when there is no explicit cursor for the statement or when PL/SQL encounters a SELECT statement that returns just one row and also when Data Manipulation Language (DML) statements like DELETE, INSERT and UPDATE are encountered. 

Some of the PL/SQL attributes that are used frequently are mentioned below:

• %ROWCOUNT: It Returns the number of rows influenced by an UPDATE, DELETE or an INSERT statement.
• %NOTFOUND: It Returns TRUE if an UPDATE, DELETE or an INSERT statement influenced zero rows or a SELECT INTO statement returned no rows. Else, it Returns a FALSE.
• %FOUND: It Returns TRUE if an UPDATE, DELETE or an INSERT statement influenced one or more rows or a SELECT INTO statement returned one or more rows. Else, it returns FALSE.
• %ISOPEN: It Returns FALSE for Implicit cursors as Oracle closes the PL/SQL cursor by default after processing its associated PL/SQL statement.

Explicit Cursors

Explicit cursors are used if you need to have better control over the Context Area via Cursor. They should be used when you are executing a SELECT statement query that will return more than one row. Cursors can process one record at a given point of time even though it stores more than one record. An explicit cursor is defined in the declaration section of the PL/SQL Block. Explicit Cursor and Implicit Cursor both have the same output, but accessing is different.

Syntax For An Explicit Cursor

Cursor Cursor-Name IS Select-Statement;

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Cursor Cursor-Name IS Select-Statement;

Processing an Explicit Cursor involves the following steps:

1. Initialize the Cursor: Initializing the Cursor also means the Declaration of the cursor in the Declaration Section of a PL/SQL Block. The name of the cursor requires to be defined along with the SELECT Statement.

Example

2. Open the Cursor: Opening the Cursor also means allocating the memory for the Cursor in the Context Area which thereby makes it sufficient to fetch and store records in it.

Example

OPEN admin_no;

1	OPEN admin_no;

3. Fetch the Cursor: Fetching the Cursor involves retrieval of data using the Fetch statement. It is used to help the Cursor process and access records or rows at a time. 

Example

4. Close the Cursor: Once the work associated for a Cursor to be completed is accomplished, it is necessary to release the Allocated Memory of the Cursor to let other tasks occupy memory with any memory down-time.

Example

CLOSE Admin_no;

1	CLOSE Admin_no;

Program to write a Cursor to list all the Administrators in the Database Table named as Admin

每当您尝试执行SQL语句时，Oracle都会在称为“上下文区域”的单独内存区域中对其进行处理。 它包含有关从数据库表/记录中检索到的信息，SQL查询，数据库所访问的行数等信息。 该上下文区域由“游标”控制。 一个PL / SQL游标可以保存一个SQL语句查询返回的一行或多行。 游标在单个时间点上保存的行集称为“活动集”。

游标基本上是指向上下文区域的指针。 PL / SQL块使用游标控制上下文区域。 游标保存由PL / SQL语句返回的一或多个行。 游标和可以命名，也可以保持匿名。 命名游标可以帮助其参考，以便稍后可以在程序中获取它，并一一处理SQL语句返回的行。

在Oracle SQL中，有两种类型的游标：

1.隐式游标2.显式游标

隐式游标

隐式游标是PL / SQL块中的默认游标。 当没有用于该语句的显式游标或PL / SQL遇到仅返回一行的SELECT语句，以及遇到诸如DELETE，INSERT和UPDATE的数据操作语言(DML)语句时，将创建它们。

下面提到一些常用的PL / SQL属性：

• ％ROWCOUNT：它返回受UPDATE，DELETE或INSERT语句影响的行数。
• ％NOTFOUND：如果UPDATE，DELETE或INSERT语句影响零行，或者SELECT INTO语句不返回任何行，则返回TRUE。 否则，它返回FALSE。
• ％FOUND：如果UPDATE，DELETE或INSERT语句影响一个或多个行，或者SELECT INTO语句返回一个或多个行，则返回TRUE。 否则，它返回FALSE。
• ％ISOPEN：当Oracle处理关联的PL / SQL语句后默认关闭PL / SQL游标时，它对隐式游标返回FALSE。

显式游标

如果需要通过游标更好地控制上下文区域，则使用显式游标。 当您执行将返回多个行的SELECT语句查询时，应使用它们。 游标即使可以存储多个记录，也可以在给定的时间点处理一个记录。 在PL / SQL块的声明部分中定义了一个显式游标。 显式游标和隐式游标都具有相同的输出，但是访问是不同的。

显式游标的语法

处理显式游标涉及以下步骤：

1.初始化游标：初始化游标还意味着在PL / SQL块的声明部分中声明游标。 游标的名称需要与SELECT语句一起定义。

例

CURSOR admin_no IS SELECT ano, aname from Admin;

1	CURSOR admin_no IS SELECT ano , aname from   Admin ;

2.打开游标：打开游标还意味着在上下文区域中为游标分配内存，从而使其足以在其中存储和读取记录。

例

3.获取游标：获取游标涉及使用Fetch语句检索数据。 它用于帮助Cursor处理和一次访问记录或行。

例

FETCH a_admin INTO a_ano, a_aname;

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FETCH a_admin INTO a_ano , a_aname ;

4.关闭游标：完成要完成的与游标相关的工作后，有必要释放游标的分配内存，以使其他任务在任何内存停机时间内占用内存。

例

编写一个游标以列出名为Admin的数据库表中所有管理员的程序

declare CURSOR c1 IS SELECT aname from Admin; rec c1%rowtype; begin for rec in c1 loop dbms_output.put_line(rec.aname); end loop; end; /

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declare CURSOR c1 IS SELECT aname from Admin ; rec c1 % rowtype ; begin for rec in c1 loop dbms_output . put_line ( rec . aname ) ; end loop ; end ; /

翻译自: https://www.thecrazyprogrammer.com/2015/07/plsql-cursors.html